Braking systems of heavy trucks generally use air under pressure as the operating medium. Typically, large air actuators have a mechanical output which applies brake pads to the brake rotor via a lever mechanism. The lever mechanism includes an operating shaft pivotable about two bearing surfaces on a common axis to urge one or more pistons against a brake pad via a cam surface. The shaft has a lever arm acted upon by the air actuator, and may include another generally shorter arm to actuate a wear adjuster mechanism. For larger brake pads, twin pistons are provided to optimize pressure distribution on the brake pad backplate. A known braking mechanism of this type is shown in document WO2004/074705 (Knorr-Bremse).
For lighter duty applications (e.g., lighter trucks or trailers of tractor-trailer units) with smaller brake pads a single piston is sufficient, but nevertheless two bearings are provided. This arrangement adds to the cost and weight of the brake, complexity of machining the bearing surfaces, and also results in problems in packaging the components within the smaller brake envelope, because space at the inboard (with respect to the vehicle on which the brake is fitted) side of the brake rotor is limited. An example of a brake of this type is EP0730107 (Perrot Bremsen).
The present applicant in EP1852627 B1 proposed a solution to this problem. However, the actuation mechanism disclosed therein retains a relatively complex wear adjuster mechanism, cover plate, carrier and pad construction.
With reference to the prior art, most air actuated disc brake units currently have a requirement for a separate cover plate facing the rotor to enable the actuation and adjustment mechanisms to be assembled therein. Locating the cover plate requires the drilling and tapping of separate holes in the caliper housing. More machining is required to manufacture the unit to allow for this and a relatively large housing is needed. Hence, the brake unit may be heavier and expensive to produce. In most applications such an arrangement is entirely satisfactory. However, the inventors have recognized that providing a brake without the requirement for a separate cover plate may result in a less costly, lighter, smaller/more compact unit.
In other prior art documents the caliper housing is formed from two cast parts, the internals of the caliper are assembled to one of the parts, and the two parts then bolted together. However, the bolted connection is an area of possible weakness that requires sealing to prevent ingress of contaminants.
The present invention seeks to overcome or at least mitigate the problems associated with the prior art.